When I have a case which has already run in parallel, and thus a time folder (eg 0.156) in the processor-subdirs, running decomposePar on the case (with only a 0 time folder) fails to remove these time folders.

1) Run OpenFOAM-5.x/tutorials/heatTransfer/chtMultiRegionFoam/multiRegionHeater with ./Allrun
2) Interrupt
3) Check whether there is a time folder in processor0 (for me it was 10) and none in the case folder itself (for me it has reconstructed, but I removed it manually)
4) decomposePar -allRegions -force, which should decompose time 0
5) Time folder 10 is still in processor folder

Giving that it works fine for the cavity case, it is probably connected to the allRegions argument.